for my 180 gallon tank 6x 2x 2 feet I have a piece of acrylic that's about 24" x 12" and covers 1 of the 2 tank openings at the top. I catch all the fish I need during water change. I take enough water out that the water level is below 12". Than I try to get the fish I want to catch to 1 end of the tank and slide the piece of acrylic into the tank. Now the fish only has a foot to swim instead of 6 feet and I can catch it very easily.